Are you crazy for working out while sick?

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Working out while ill may not be the best idea.

Story highlights

You can work out if you're mildly sick, because it can stimulate your immune system

Workouts should be at home, because you can spread the cold or flu through your sweat

If you have a flu or fever, stay home, rest and drink lots of liquid

(Life by DailyBurn)Even if you're psyched to work out, let's be honest, sometimes your body is just not having it. We're all for pushing yourself when it's laziness that's holding you back, but there are times when heading to spin class or the weight room can do your body more harm than good.

"Health comes before fitness, and even though we can't control when we get sick, we can do our part in helping our body recover from illnesses," says certified personal trainer Idalis Velazquez, owner of IV Fitness in Florida. And while rest is a staple of many feel-good regimens, in other cases, sweating it out can actually help your health, she says.

Whether you've got the sniffles or the full-on flu, here's what you need to know about when you should hit the gym — or if you're better off crawling under the covers.

You have a cold

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If you don't have a fever, a runny nose is no reason to skip a workout, says Dr. Wayne Stokes, director of sports medicine at NYU Langone Medical Center's Rusk Rehabilitation. Instead, scale your intensity. Restless nights can stunt muscle strength and a stuffed-up nose can make cardio a struggle. "If you are mildly sick, [staying] active will promote your immune function, and help you sleep better," he says.

"When I have a mild cold or stuffy nose ... I don't push myself or place any higher demand on my body during those sessions. I treat it like an active recovery day and I tell my clients to do the same," Velazquez adds.

However, gym etiquette does dictate you should keep your workout quarantined to your home.

When you have a cold, you're typically contagious for about five full days, and your germs spread most easily during your two to three most symptomatic days, says Dr. Michael P. Angarone, assistant professor of infectious diseases and medical education at Northwestern University's Feinberg School of Medicine.

Cold germs can live on hard surfaces like dumbbells for hours. And, get this, even if you wash your hands, sweat can carry snot particles down your face and onto equipment. (Insert the heebie jeebies here.)

Unless you count sprinting to the bathroom as a workout, you'll need to hold off until you feel better.

Diarrhea and vomiting — frequently caused by the norovirus (aka the stomach flu) — can result in severe fluid loss and dehydration, according to Angarone. Breaking a sweat can further exacerbate fluid loss, leading to symptoms ranging from dry skin to a rapid heartbeat. While you might need one bottle of water to fuel you during a regular workout, it could take three or more to keep you hydrated when you are suffering from stomach issues. And there's no good way to know how much you need, he says.

What's more, the norovirus and other stomach-churning germs are highly contagious and can live on hard surfaces until unsuspecting gym-goers pick them up, Angarone says. If you feel well enough to keep moving despite your symptoms, Stokes says stretching (at home) is safe. Easing any muscle tension with a foam roller might also help you feel better.

You have a fever or the flu

Take medicines and stay in bed, please. Working out with a fever can make your health way worse, Stokes says. That's largely because a fever, like vomiting, can cause workout-wrecking dehydration.

Even more concerning, high temperatures (101 degrees and up) have been linked to heart damage. And exercising through a fever can raise your risk of myocarditis, an inflammation of the heart muscle that may result in heart dysfunction, failure or sudden death, Stokes says. "It's not common, but it is possible and good reason not to push yourself."

Angarone recommends resting until your fever has been gone for a full 24 hours, without the help of any fever-reducing medications like ibuprofen. If you have the flu, it will likely take three to five days for your symptoms to let up. Don't forget that your fever, muscle aches and other pains are often signs that your body is trying to fight off a virus. So, if you make your body split its energy and resources between the infection and exercise, you will likely be sick longer, he says.

Even a week of bed rest will cause a slight loss of muscle mass and strength, according to Stokes. So when you head back into the gym, don't be discouraged if it takes a few workouts to get back to where you used to be.

Remember, listen to your body, and if you've got a fever or a stomach bug, be sure to rest. Contagious folks should work out at home — and if you're fighting a cold, a foam roller might be your new best friend.